WASHINGTON - The Obama administration has taken the extraordinary step of authorizing the targeted killing of an American citizen, the radical Muslim cleric Anwar al-Awlaki, who is believed to have shifted from encouraging attacks on the United States to directly participating in them, intelligence and counterterrorism officials said Tuesday.

Mr. Awlaki, who was born in New Mexico and spent years in the United States as an imam, is in hiding in Yemen. He has been the focus of intense scrutiny since he was linked to Maj. Nidal Malik Hasan, the Army psychiatrist accused of killing 13 people at Fort Hood, Tex., in November, and then to Umar Farouk Abdulmutallab, the Nigerian man charged with trying to blow up a Detroit-bound airliner on Dec. 25.

American counterterrorism officials say Mr. Awlaki is an operative of Al Qaeda in the Arabian Peninsula, the affiliate of the terror network in Yemen and Saudi Arabia. They say they believe that he has become a recruiter for the terrorist network, feeding prospects into plots aimed at the United States and at Americans abroad, the officials said.

It is extremely rare, if not unprecedented, for an American to be approved for targeted killing, officials said. A former senior legal official in the administration of George W. Bush said he did not know of any American who was approved for targeted killing under the former president.

'Direct actions' But the director of national intelligence, Dennis C. Blair, told a House hearing in February that such a step was possible. “We take direct actions against terrorists in the intelligence community,” he said. “If we think that direct action will involve killing an American, we get specific permission to do that.” He did not name Mr. Awlaki as a target.

The step taken against Mr. Awlaki, which occurred earlier this year, is a vivid illustration of his rise to prominence in the constellation of terrorist leaders. But his popularity as a cleric, whose lectures on Islamic scripture have a large following among English-speaking Muslims, means any action against him could rebound against the United States in the larger ideological campaign against Al Qaeda.

The possibility that Mr. Awlaki might be added to the target list was reported by The Los Angeles Times in January, and Reuters reported on Tuesday that he was approved for capture or killing.

“The danger Awlaki poses to this country is no longer confined to words,” said an American official, who like other current and former officials interviewed for this article spoke of the classified counterterrorism measures on the condition of anonymity. “He’s gotten involved in plots.”

The official added: “The United States works, exactly as the American people expect, to overcome threats to their security, and this individual — through his own actions — has become one. Awlaki knows what he’s done, and he knows he won’t be met with handshakes and flowers. None of this should surprise anyone.”

As a general principle, international law permits the use of lethal force against individuals and groups that pose an imminent threat to a country, and officials said that was the standard used in adding names to the list of targets. In addition, Congress approved the use of military force against Al Qaeda after the Sept. 11, 2001, terrorist attacks. People on the target list are considered to be military enemies of the United States and therefore not subject to the ban on political assassination first approved by President Gerald R. Ford.

Both the C.I.A. and the military maintain lists of terrorists linked to Al Qaeda and its affiliates who are approved for capture or killing, former officials said. But because Mr. Awlaki is an American, his inclusion on those lists had to be approved by the National Security Council, the officials said.

At a panel discussion in Washington on Tuesday, Representative Jane Harman, Democrat of California and chairwoman of a House subcommittee on homeland security, called Mr. Awlaki “probably the person, the terrorist, who would be terrorist No. 1 in terms of threat against us.”